1. Field of the Invention
The present invention relates to wireless communications, and more specifically to a method and apparatus for generating a precoding matrix in a wireless communication system.
2. Related Art
MIMO technology can be used to improve the efficiency of data transmission and reception using multiple transmission antennas and multiple reception antennas. MIMO technology may include a space frequency block code (SFBC), a space time block code (STBC), a cyclic delay diversity (CDD), a frequency switched transmit diversity (FSTD), a time switched transmit diversity (TSTD), a precoding vector switching (PVS), spatial multiplexing (SM) for implementing diversity. An MIMO channel matrix according to the number of reception antennas and the number of transmission antennas can be decomposed into a number of independent channels. Each of the independent channels is called a layer or stream. The number of layers is called a rank.
The dirty paper coding scheme, one of MIMO technology, can reduce interference by removing other user's data which act as interference in advance from a transmitting unit, and is known to provide, in theory, maximum channel capacities in MIMO system. Reference can be made to M. H. M. Costa, “Writing on Dirty Paper,” IEEE Trans. Inf. Theory, vol. 29, no. 3, pp. 439-441, May 1983, for the technology. The dirty paper coding scheme, however, is difficult to implement in practical system since the scheme requires lots of channel information and presents much complexity in calculation due to the nature of non-linear calculation. Therefore, various alternative methods that can implement the principle of dirty paper coding scheme by using linear calculation have been proposed. Among various alternative methods, a zero-forcing beamforming (ZF-BF) method uses the pseudo inverse matrix of a channel as a precoding matrix of a base station, taking advantage of the property that unit matrix is obtained by multiplying the channel with the precoding matrix. In other words, user pre-processing is carried out in the base station taking consideration of the interference between users. The ZF-BF method has the merit of relatively low complexity in calculation and ease of actual implementation through the use of only linear calculations. Also, the ZF-BF method has the same growth rate of sum rate as that of the DPC method. Since the growth rate of sum rate has meaning only when the number of users is large, however, various methods such as a multiple reception antenna method, a multiple mode transmission method and a multiple user scheduling method have been proposed for improving performance when the number of users is small.
Meanwhile, deterioration in performance can occur in the ZF-BF method when signal-to-noise ratio (SNR) is low. The reason is that noise enhancement occurs when SNR is low since precoder is designed by removing interference signal of other users at the transmission end in the ZF-BF method. To solve this problem, a regularized channel inversion (RCI) method has been proposed. The precoding matrix generated by the conventional ZF-BF method and RCI method can be expressed as follows by Equation 1.
                                          G            ZF                    -                                                                      α                  ZF                  ′                                ⁡                                  (                                                            V                      H                                        ⁢                    V                                    )                                                            -                1                                      ⁢                          V              H                                      ⁢                                  ⁢                              G            RCI                    =                                                                      α                  RCI                  ′                                ⁡                                  (                                                                                    V                        H                                            ⁢                      V                                        +                                                                  K                        ρ                                            ⁢                                              I                        K                                                                              )                                                            -                1                                      ⁢                          V              H                                                          〈                  Equation          ⁢                                          ⁢          1                〉            
In Equation 1, V=[V1T, V2T, . . . , VKT]T is channel direction information (CDI) which is fed back, α′ZF and α′RCI are normalization constants for fulfilling the transmission power constraint of the base station, and ρ represents SNR. In other words, the precoding matrix GRCI generated by the RCI method is generated considering the generated SNR, thereby solving the problems of the ZF-BF problems to some degree.
The ZF-BF method and the RCI method described above assume the case of perfect channel, and presents optimal performance when SNR is high. When channel error exists, however, the ZF-BF method and the RCI method have problem in performing optimally. Therefore, there needs to propose methods of precoding that can provide improved performance when channel error exists.